The vast majority of vehicles currently in use incorporate vehicle communication systems for receiving or transmitting signals. For example, vehicle audio systems provide information and entertainment to many motorists daily. These audio systems typically include an AM/FM radio receiver that receives radio frequency (RF) signals. These RF signals are then processed and rendered as audio output.
Vehicle video entertainment systems are gaining in popularity among motorists who want to provide expanded entertainment options to rear seat passengers, such as children. Rear seat passengers in vehicles equipped with video entertainment systems can watch movies or play video games to pass time during lengthy trips.
Some vehicle video entertainment systems incorporate tuners capable of receiving broadcast signals in the VHF and UHF frequency bands. Such systems allow passengers to watch broadcast television, further expanding their entertainment options. However, programming is limited to local broadcast stations. In addition, picture and sound quality is limited by the analog nature of the broadcast signals. Further, signal quality may be poor in some areas, such as remote locations.
Satellite-based broadcast systems, such as Direct Broadcast Satellite (DBS), provide subscribers with digital television programming. Because the signals used by DBS systems are digital, picture and sound quality is enhanced relative to traditional analog broadcasting systems. In addition, a DBS transmitter can provide coverage for a much larger geographic area than the terrestrial-based transmitters used by analog broadcasters. For example, it is possible to travel across a large portion of the United States without needing to change channels as different metropolitan areas are entered and exited.
Some conventional DBS receivers use a circularly polarized microstrip linear or planar array antenna to receive satellite signals. Circular polarization has long been used for communications, among other applications. Circular polarization is particularly well-suited for applications involving mobile receivers because the orientation of a circularly polarized signal does not change relative to the receiver as the receiver moves. Circularly polarized microstrip linear array antennas are preferably characterized by high efficiency, small beam walking, and good axial ratio at the satellite look angles.
Conventional microstrip linear array antennas include the rampart line linear array antenna and the loop line linear array antenna. Both of these types of microstrip linear array antennas suffer from large losses and extensive beam walking. Another conventional type of microstrip linear array antenna, known as the herringbone linear array antenna, does not provide a good axial ratio off its mechanical boresight.